Switching devices, in particular power switches, are used for amongst other things safely switching off power in the event of a short circuit, thus protecting loads and installations. Electrical or mechanical switching devices are also suitable for operationally correct manual switching of loads and for safe disconnection of an installation from the power grid during servicing work or modifications to the installation. Electrical switching units are often operated electromagnetically.
As a consequence, switching units of this type are highly technical electrical switching devices with integrated protection for motors, cables, transformers and generators. They find their application at functional locations with low switching frequency. In addition to short-circuit protection, switching units of this type are also suitable for overload protection.
In the event of a short circuit, an electrical switching unit safely switches off power to an electrical installation. It thus offers fuse protection against overload. Any cable through which the current flows heats up more or less strongly, depending on the ratio of the magnitude of current to the cross-sectional area of the conductive cable, which is known as the current density. The current density must not be too large, since otherwise the cable insulation may become charred, or
a fire may break out, as a result of the excessive heat. In order to protect electrical installations against these damaging effects, switching units are used as overcurrent protection devices.
Power switches comprise two independently acting trip mechanisms, connected in series, for overload protection and for short-circuit protection. The protection against short circuits is effected by an electromagnetic trip that operates with almost no time delay. In the event of a short circuit, the electromagnetic trip unlatches a switch lock of the power switch without delay. A switch armature disconnects the contact element before the short-circuit current can reach its maximum value.
Known switching units include a sliding contact unit having a sliding contact and a moving contact element. The moving contact element is itself formed of electrical contacts. Switching units of this type also include first contacts to an electric cable. In a switched-on state, the electrical contacts of the movable contact element contact the fixed contacts of the switching unit. In the event of a short circuit, the electrical contacts of the movable contact element are released from the fixed contacts, so that the flow of current is interrupted. The movable contact element is thus released from the fixed contacts.
Known sliding contacts of switching units frequently comprise two guidance systems, an inner guidance system and an outer guidance system. The outer guidance system is used when the switching procedure, in other words switching on or switching off, is effected by means of a switch lock of the switching unit. This switching of the power state is effected in a controlled manner. The inner guidance system, on the other hand, is used in the event of a short circuit, when the switching procedure is effected by a switching armature, commonly a plunger in combination with a guidance pin, of the switching unit.
Accordingly, when switching off of power as a result of a short circuit, the movable contact element moves quickly along the inner guidance system in front of the sliding contact, strikes impact surfaces in what is known as the lower part of the switching unit, and flies back again along the inner guidance system. In such implementations it flies against the switching armature or the guidance pin of the switching unit.
If heavy short circuits occur, large magnetic forces in turn develop between the movable contact element and the fixed contact elements. These are, in part, the current loop forces between the fixed contact elements and the bridge. There are also large current magnitude forces between the silver contacts. The effect of these two forces is that in the event of a short circuit, the bridge is thrown suddenly against its resulting spring force and strikes against the impact dome in the lower part. Since it is not possible for the impact dome to be positioned in the center of the switching chamber (as this location is required by the guide plate), it has in known constructions been divided and located against the chamber walls. Although the bridge is secured against rotation by a guide pin, its guide play cannot however be adequately limited. If the bridge makes use of the degrees of freedom that it has as a result of its construction, it might only meet the impact dome on one side, and then become wedged in the lower part.
A further problem with these known constructions is that when certain switching activities are carried out at switching devices, in particular at power switches, extreme heating of the switch contact occurs. In particular in the case of changing to the off position after a period of being switched on, the very hot contact bridge can damage the surrounding areas of the housing of the switch chamber, which are usually formed of plastic. This mainly occurs in the off position since, in this case, the thermal losses arising in the bridge cannot flow away into the fixed contact elements, and when in the off position the bridge is in a position that is almost thermally insulated on every side. Since the bridge must be freely movable, it has a certain degree of rotary play in the axial direction of the sliding contact to the switching chamber wall. Under unfavorable conditions, the bridge uses its rotary play and becomes located too close to the plastic wall of the switching chamber; then, as a result of the hot bridge, the plastic swells and surrounds the bridge, as a result of which the power switch is destroyed.